This invention relates to a motor vehicle suspension control including a solenoid actuator, the solenoid actuator comprising an actuating coil connected in series with a transistor across an electric power source. It more particularly relates to such a control including protective means for the transistor if a short circuit develops across the actuating coil.
This invention is also concerned with minimizing the cost of the vehicle suspension control by combining circuit functions, where possible, in the same circuitry. One of the functions of a vehicle suspension control of the type contemplated is vehicle leveling or height control, in which the control responds to a signal from a vehicle height sensor to actuate apparatus in order to change the height of all or a portion of the vehicle. In order that the control not respond to transient vehicle body movements, a time delay is generally observed for any out of limit height signal before action is taken.
In addition, with regard to the protection of the transistor which controls activation of the actuating coil of the solenoid actuator, it is known that the current in the transistor will be indicated by the voltage across it, and that this can be the basis of a protective circuit. A complication, however, with this approach, is that the voltage across the transistor when it is cut off is sufficiently similar to the voltage when current levels are too high that some confusion may occur. Therefore, when the transistor is first turned on, a short time delay is allowed to occur before the voltage across the transistor is looked at as a feedback signal. This time delay allows the transistor to begin conducting and go into saturation if no circuit errors are present, so that a voltage across the transistor higher than the saturation voltage may be reliably assumed to be indicative of overcurrent and thus a circuit error rather than no current.
Finally, if a voltage across the transistor seemingly indicating out of saturation is detected, the activation of the solenoid is stopped. However, since the voltage may be due to some transient condition, it is desirable to try the activation again after a suitable time delay and to continue to retry, if each is unsuccessful, for a number of such time periods, before giving up. All these time delays provide an opportunity for consolidation of circuitry in a clocked digital system.